1. Field of the Invention
The present invention relates to an optical scanning apparatus and an image forming apparatus using the same. The present invention is suited to an image forming apparatus such as a laser beam printer, a digital copying machine, or a multi-function printer, which adopts an electrophotography process.
2. Description of the Related Art
Conventionally, in an optical scanning apparatus that is used for a laser beam printer or a digital copying machine, a light flux emitted from a light source unit is guided to a light deflecting device by an incident optical system.
In such optical scanning apparatus, high speed and high resolution can be achieved by increasing the number of light emitting portions of the light source unit.
There are conventionally proposed various optical scanning apparatuses that can achieve high speed and high resolution by increasing the number of light emitting portions of the light source unit (see Japanese Patent Application Laid-Open No. H09-26550, and Japanese Patent Application Laid-Open No. 2001-154128).
Japanese Patent Application Laid-Open No. H09-26550 discloses a technology for improving optical performance among a plurality of light fluxes by arranging a plurality of light emitting portions in a symmetric manner with respect to an optical axis of a collimator lens.
Japanese Patent Application Laid-Open No. 2001-154128 discloses a technology of adjusting a light source and a laser for improving optical performance among a plurality of light fluxes emitted from a plurality of light emitting portions on a surface to be scanned.
In the conventional optical scanning apparatuses described above, if a multibeam light source unit including a plurality of light emitting portions at positions far from the optical axis of an incident optical system in a main scanning direction is used for a light source unit, a focal position on the surface to be scanned of each light flux emitted from each light emitting portion of the multibeam light source unit is shifted so that a difference of spot diameter occurs between the light fluxes. Thus, there is a problem of deterioration of an image.
In addition, it is necessary to increase the number of lenses of the incident optical system so that the difference of the spot diameter between light fluxes does not occur. This causes not only problems of upsizing of the entire apparatus and complication thereof but also problems that sensitivity of the incident optical system is increased, and hence a performance deterioration due to a manufacturing error is increased.
It is supposed to use a light source unit including two light emitting portions having different distances from the optical axis of the collimator lens in the main scanning direction (due to designing and manufacturing errors) as the light source unit including a plurality of light emitting portions that are spaced apart from one another in the main scanning direction.
In this case, if a shape of a lens surface of the collimator lens forming the incident optical system is circular in the main scanning section, a field curvature in the main scanning direction occurs in the lens surface of the collimator lens.
In other words, a difference of condensing state in the main scanning direction occurs between two light fluxes that have passed through the lens surface of the collimator lens.
For example, a difference of parallelism in the main scanning direction occurs between two collimated light fluxes that have passed through the lens surface of the collimator lens.
The focal positions of the two light fluxes emitted from the two light emitting portions on the surface to be scanned differ from each other. As a result, the spot diameters of the two light fluxes on the surface to be scanned are different from each other, and this causes an image quality difference between images based on the two light fluxes emitted from the two light emitting portions.